Modbus is a messaging protocol that defines the content of that message.
In a short time hundreds of vendors implemented the Modbus messaging system in their devices and Modbus became the de facto standard for industrial communication networks.
Communication between the Modbus nodes was achieved with messages. It was an open standard that described the messaging structure. The physical layer of the Modbus interface was free to choose.
The original Modbus interface ran on RS232, but most later Modbus implementations used RS485 because it allowed longer distances, higher speeds and the possibility of a true multi-drop network. RS485 is hardware serial-interface standard that defines the hardware for carrying a message.
If you need more info on MODBUS check this manual out!